Circuit breaker and water question

[livoniaben]

I have an 8 year old KZ-SPREE travel trailer.

There is an in-line device ab out 18" into the wire going to the + side of my house battery. It is pretty corroded and I can make out "30AMP" on the side of it, I think it is a circuit breaker. I want to replace it but --
What does it do/protect?
Are there different kinds of 30AMP CB's and which kind do I need?

Also, have only used the fresh water holding tank once when the trailer was new; always used city water connection since then.
If I fill the water tank will this also fill the water heater??
If so, will the water heater stay full as long as there is water in
the water tank??

Thanks for responding to questions I know I should already have the answers to but hey -- I'm 81 now and getting a little forgetful, too much stuffed into this old brain.

[SoundGuy]

livoniaben wrote:
There is an in-line device ab out 18" into the wire going to the + side of my house battery. It is pretty corroded and I can make out "30AMP" on the side of it, I think it is a circuit breaker. I want to replace it but --
What does it do/protect?
Are there different kinds of 30AMP CB's and which kind do I need?

It's just a garden variety resettable circuit breaker, wired inline with the battery positive cable and intended to disconnect 12 vdc power entirely from the trailer should a problem arise.

If I fill the water tank will this also fill the water heater??
If so, will the water heater stay full as long as there is water in
the water tank??

Sure, if you turn the pump on and purge the system of air by opening a couple of faucets until the flow of water is steady.